Investigation of teratogenic mechanisms in rats and rabbits will continue. Emphasis will be on the identification and quantitation of the early changes that are demonstrable in the embryo a few hours after teratogenic treatment. Particular effort will be made to localize such effects in regions known later to be the sites of malformation. Such known teratogenic agents as hydroxyurea, acetazolamide, thalidomide, cytosine arabinoside and aspirin will be used in these studies. The underlying hypothesis is that differential sensitivity of localized groups of embryonic cells or tissues to the deleterious actions of teratogenic agents is the basis of most induced teratogenesis.